In the past thirty years, membrane bioreactor (MBR) combining membrane separation and biotreatment processes together has been attracted broad attention from domestic and overseas research institutions. The external pressure is needed to be the driving force of permeate in traditional MBRs, such as reverse osmosis, nano-filtration, ultra-filtration, and micro-filtration etc. Although these external pressure driving membrane separation processes have advantages, the energy consumption is huge due to the application of external pressure, meanwhile, heavy membrane fouling is often accompanied with these membrane separation processes, leading to lower operating efficiency. All mentioned above are main restrictions of large scale application of MBR in wastewater treatment field.
To solve problems mentioned above, Cornelissen developed an osmosis membrane bioreactor (OMBR) using osmotic pressure as driving force for the first time. In OMBR, the forward osmosis (FO) membrane is adopted as separation medium and the permeate is driven by osmotic pressure instead of external pressure, and the membrane fouling is greatly reduced comparing to these membrane processes using external pressure because operating pressure is decreased. Removal rate of COD and NH4+—H of OMBR system could reach more than 95% to obtain high quality effluent. Besides, high recovery rate by hydraulic cleaning and retention rate of pollutants of the OMBR system are the incomparable advantages comparing to other membrane separation processes. Therefore, research on theory and practical application of OMBRs shows a promising future to break through the bottlenecks of traditional MBRs such as high energy consumption, heavy fouling etc., and OMBR process is considered to be an advanced technology with high efficiency for domestic water treatment.
In the OMBR system, water molecule is driven into draw solution side by high osmosis pressure through FO membrane from sludge mixed liquid side with low osmosis pressure, and the diluted draw solution is recycled by reverse osmosis or other physical or chemical separation methods. Recent years some reports have verified the higher efficiency and feasibility of OMBR process, which provides sound theoretical foundation for its practical application.
However, during practical application, it has been found that the actual membrane flux is lower than theoretical flux in FO membrane system by using osmosis pressure difference as driving force, which is mainly due to the existence of concentration polarization especially the internal concentration polarization. Therefore, it's important to optimize the structure of membrane modules in order to mitigate the negative effects of internal concentration polarization on membrane permeation flux. Hanmin ZHANG etc. have provided a modified FO membrane module and application thereof in which metallic mesh is displaced between two FO membranes with more uniform water distribution, while secondary pollution is brought because metallic ions was released into membrane chamber and liquids during the long-term application (Chinese Patent Application Publication No: CN103801195A). Zhong REN etc. have provided FO membrane modules and subassembly thereof and a flat plate is used to divide membrane chamber into upper flow region and bottom flow region, however, dead angles of flow regime are easily generated in this module (Chinese Patent No: CN204159217U).
To conclude, the present disclosure addresses the higher convenience and efficiency method to mitigate the concentration polarization of FO membrane.